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Non-native plant species in Mediterranean islands 2569

and ports are widely recognized as being preferential Furthermore, a greater proportion of volcanic
centers of introduction of non-native taxa (Botham lithology seems to promote the establishment of a
et al. 2009). Several papers have also emphasized the higher number of non-native species on small islands
role of roads as conduits for non-native plant dispersal impacted by human disturbance. Differences in
(Von Der Lippe and Kowarik 2007); moreover, roads lithology determine differences in soil properties
tend to fragment natural communities, which weakens (e.g. water and nutrient availability), thus affecting
the capacity of ecosystems to recover from distur- the growth and seedling regeneration possibilities of
bance events (Saunders et al. 1991). A case in point is non-native species (Rodgers and Parkel 2003). In this
one of the residual populations of Limonium sommie- view, volcanic islands are often characterized by
rianum (Fiori) Arrigoni, a species that is endemic to fertile and deep soils that might enhance the richness
three small islands in the Tuscan Archipelago. The of introduced plant species, providing suitable condi-
survival of L. sommierianum on the island of Giglio is tions even for species that are relatively less tolerant to
currently threatened by the expansion on rocky drought. Moreover, volcanic islands are likely to have
habitats of Carpobrotus edulis (L.) N.E.Br., which been agriculturally exploited to a greater degree
has been cultivated along the road leading to than calcareous islands and consequently to contain
a relatively isolated lighthouse. The invasion of a higher number of non-native weeds.
C. edulis is aided by the wild rabbit (Oryctolagus Our analysis did not reveal any effect of the
cuniculus L.), which disperses its seeds. This animal proportion of protected island area on non-native
was introduced to the island of Giglio from Sardinia in species richness. However, the presence of natural
1945 as game for hunters; repeated attempts have, reserves does not necessarily ensure low levels of
however, been made since its introduction to eradicate invasion but may actually act as a factor that promotes
this animal from the island because of the serious tourism, as is suggested by the finding that the number
damage it has inflicted on both crops and natural of tourists that visit a protected area is directly related
systems. The positive interaction between C. edulis to the number of non-native species recorded (Vila ` and
and O. cuniculus represents one example of invasional Pujadas 2001). It should, nevertheless, be borne in
meltdown, which has previously been investigated on mind that Italian small island natural reserves are not
French Mediterranean islands (Bourgeois et al. 2005). subjected to high tourist pressure.
Lastly, some geographical and environmental factors With regard to differences in the composition of
affect the richness of introduced plant species. For total total non-native and established non-native species,
non-native species, the most influential of these char- the results of the variation partitioning highlight
acteristics is the latitudinal variation, a proxy of the varying effects of the geographical, environmental
climatic gradient, which acts in those islands in which and human-mediated components. Although this sta-
the effect of human activities is absent or low. Although tistical method does not necessarily identify causal
Pys ˇekandRichardson(2006) did not detectanyeffectof relationships, it does shed light on the processes that
latitude on the degree of invasion of islands worldwide, may have generated the observed patterns (Borcard
it is possible that, on our scale of observation, the et al. 1992). The variation in total non-native species
warmer climate that characterizes the southernmost composition on the islands considered here was
islands would limit the water availability and create affected above all by environmental variables. These
unfavourable environmental conditions for the growth findings point to the role played by area and altitude in
and survival of introduced plants. Climate matching increasing the chances for a range of non-native plant
between the native and the introduced range plays a species to colonize a variety of ecological conditions.
fundamental role in promoting plant invasions (Thuiller Larger islands generally contain greater habitat heter-
et al. 2005); moreover, climate is one of the physical ogeneity and can consequently support a wider range
factors that most affects the success of non-native of non-native plant species (Whittaker and Ferna ´ndez-
species in the colonization and establishment phases Palacios 2008). An island’s maximum elevation has
(Theoharides and Dukes 2007). Indeed, habitats char- also frequently been correlated with its habitat diver-
acterized by high levels of stress are already considered sity (Triantis et al. 2008).
to be less prone to invasions, and to host lower numbers With regard to the variation in the composition
of introduced plant species (Carboni et al. 2010a, b). of established non-native species, our study shows a

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